1. Field of the Invention
The present invention relates to a differential amplifying circuit, and more particularly, to an amplifying circuit of current subtraction type where a current based on a difference between output currents of 8 pair of transistors constituting a differential amplifier is obtained as an output.
2. Description of the Prior Art
FIG. 1 shows a conventional amplifying circuit. A differential amplifier 1 having a constant current source 2 is constituted by a pair of transistors Q1 and Q2. An output of the differential amplifier 1 is provided from input-side transistors Q3 and Q5 of first and second current mirror circuits 3 and 4 to a third current mirror circuit 5 through output-side transistors Q4 and Q6 of the first and second current mirror circuits 3 and 4.
At this time, an output of the transistor Q6 is coupled to the base and collector of an input-side transistor Q7 of the third current mirror circuit 5, while an output of the transistor Q4 is coupled to the collector of an output-side transistor Q8 of the third current mirror circuit 5. An output of the amplifying circuit of FIG. 1 is directed from a connection (a) of the collector of the transistor Q4 and the collector of the transistor Q8 to an output terminal 6. The output current is the difference between output currents I3 and I4 of the transistors Q4 and Q6 which currents correspond to differential output currents I1 and I2 of the differential amplifier 1.
That is, since the output current I4 of the transistor Q6 also flows to the collector of the transistor Q8 through working of the current mirror circuit 5, both the current I3 which flows from the transistor Q4 and the current I4 which flows to the transistor Q8 are present at the connection (a). A current corresponding to the difference between the currents I3 and I4 flows through the output terminal 6.
Since I1=I2 and I3=I4 when no alternating current inputs are present at input terminals A and B of the differential amplifier 1, the output current passing through the output terminal 6 is 0.
In a case where this circuit is used for, for example, an operational amplifier, an output is partly fed back toward the input side as shown by a dotted line 7. At this time, oscillation may occur on a high-band H of frequency characteristics shown in FIG. 3. To avoid this, a capacitor C1 is connected between the base and collector of the transistor Q4, and a resistor R1 is connected to the base of the transistor Q4. The resistor R1 and the capacitor C1 serve as a phase compensating circuit which makes phase compensation to prevent the occurrence of the oscillation on the high-band H.
In the above-described conventional amplifying circuit, since the resistor R1 is provided, the static output currents I3 and I4 are unbalanced. Consequently, a condition where the output current is not 0 under a static condition (that is, an offset) is generated. Since it is clear that this results in an amplification error, the amplification accuracy of the amplifier deteriorates.